CODEWITHSUNDEEP
java
Calicoder
Calicoder

Reputation: 1452

How can you tell if a JDK class is implemented natively instead of in java code?

For example, when exploring source classes in IntelliJ, and you find a class like Constructor in java.lang.reflect...The parameters to all the methods are var0,var1,var2,...varN. But other than that, how would one know it is implemented natively?

EDIT: The correct question really is how to tell whether a method is native in Java, because, as pointed out by responders, classes themselves can't be "native."

Upvotes: 0

Views: 94

Answers (1)

barteks2x
barteks2x

Reputation: 1355

In general, classes themselves are never native. Only methods can have native implementation.

But that doesn't mean the JVM can't treat some classes in special way. But you shouldn't be able to tell the difference. The JVM treats them differently as an optimization (for example String class) or because doing otherwise would be very inconvenient (for example NoClassDefFoundError is treated in a special way, and can be thrown and printed even if the class doesn't exist on your classpath. You can try it by moving/renaming rt.jar and running java).

As for the constructor arguments being named varN, it's likely because you didn't actually install JDK source, and whatever IDE you use shows you decompiled java code, which doesn't have variable names preserved. In the case of Constructor class, the constructor is likely to be actually implemented in java, but called, and the object returned, from native code.

A method is native if it has native modifier. But there are also methods that JVM treats as intrinsics. The only way to know those, together with special classes, for sure is to look at JVM source. There is also a very thin line between what can be considered JVM optimization and what can be considered intrinsic, and because you don't normally modify JVM code, so unless you are JDK developer, it doesn't really matter. This is also an implementation detail that can change in different JVM or different JVM version. In fact, there are attempts at writing JVMs completely in java, without any real native code, aside of some tiny startup wrapper. In those cases, everything is in java. There is no native code.

Upvotes: 2

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